test(server-utils): Remove postgres.js unit test that replicated SDK async-context logic (#22097)

Removes the postgres.js unit test that stubbed the SDK's async context
strategy to drive spans, which replicated SDK internals in the test
layer. Per the review thread in #21885, we rely on the integration tests
instead.

The orchestrion path is already covered end to end by the `postgresjs`
node integration suite. The remaining unit-only assertions checked
internal implementation details (not re-creating a span on a re-entrant
call, skipping an already-wrapped query, no span without an active
parent) rather than the spans a user actually sees, so there was nothing
meaningful to move into an integration test.

Ref:
#21885 (comment)

Co-authored-by: Opus 4.8 <noreply@anthropic.com>

Author: andreiborza